MCAT Biology : Respiratory Structures and Lung Mechanics

Study concepts, example questions & explanations for MCAT Biology

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Example Questions

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Example Question #11 : Respiratory System

Which of the following processes is not involved in inhalation?

Possible Answers:

Contraction of the interior intercostal muscles

Expansion of the thoracic cavity

Expansion of the lung tissue

Contraction of the diaphragm 

Correct answer:

Contraction of the interior intercostal muscles

Explanation:

The process of inhalation involves a coordinated series of steps beginning with the contraction and flattening of the diaphragm. This serves to decrease the pressure in the thoracic space, pulling the lung with it to expand the lung volume. By the ideal gas law, we know that when the volume is increased at a fixed temperature, the pressure decreases. The low intra-lung pressure pulls air in from outside, completing the inspiratory process.

To promote forceful inhalation, the exterior intercostals can contract. These muscle are located on the outside of the ribs and help to further expand the thoracic cavity when contracted. In contrast, the interior intercostal muscles are located on the inside of the ribs and help to shrink the thoracic cavity during contraction, aiding in forceful exhalation. The interior intercostals are not involved in inhalation.

Example Question #12 : Respiratory Structures And Lung Mechanics

Which of the following cases best represents exhalation?

Possible Answers:

Diaphragm and external intercostal muscles contract

Diaphragm contracts and external intercostal muscles relax

Diaphragm and internal intercostal muscles relax

Only external intercostal muscles contract

Diaphragm relaxes and internal intercostal muscles contract

Correct answer:

Diaphragm relaxes and internal intercostal muscles contract

Explanation:

The diaphragm is a dome-shaped muscle at the base of the thoracic cavity. When contracted the diaphragm pulls downward, expanding the volume of the thoracic cavity and reducing the pressure. This negative pressure pulls air into the lungs, allowing inspiration. The external intercostal muscles are situated along the outside of the rib cage, and can help expand the ribs when contracted to cause forced inhalation.

When the diaphragm relaxes, the thoracic cavity shrinks to its normal size and releases the air from the lungs. Exhalation is mostly passive, however contraction of the internal intercostals can increase the pressure in the thoracic cavity. The internal intercostals are arranged on the interior of the rib cage, and can effectively pull the ribs closer together. This further decreases the space available to the lungs, causing forced expiration.

Example Question #12 : Respiratory Structures And Lung Mechanics

Guillen-Barre syndrome is a condition that results in ascending paralysis. If this condition becomes severe, it can cause paralysis of the diaphragm and intercostal muscles.

Which best describes the impact that this paralysis would have on respiration? 

Possible Answers:

Inability to create a negative pressure in the lungs

A reduction in tidal volume, but an increase in inspiratory reserve volume

An increase in tidal volume, but a reduction in inspiratory reserve volume

Inability to forcibly exhale air from the lungs

Correct answer:

Inability to create a negative pressure in the lungs

Explanation:

The diaphragm and intercostal muscles are used in normal respiration to draw air into the lungs. The diaphragm flattens and descends, and the intercostal muscles move the rib cage outward to increase chest volume. These actions increase the chest volume during passive inspiration (contraction) and decrease the chest volume during passive expiration (relaxation). An increase in chest volume with result in a negative pressure in the lung that acts to pull air into lungs. Paralysis of these muscles would lead to an inability to create a negative pressure in the lungs and would inhibit inspiration.

Tidal volume is determined by the total volume of air moved with each passive breath. It is the sum of inspired air and expired air. If inspiration is inhibited, this value will decrease. Inspiratory reserve volume is the additional volume that can be drawn in by forced inspiration, via voluntary contraction of the diaphragm. This value would also decrease with paralysis of the diaphragm.

Example Question #14 : Respiratory Structures And Lung Mechanics

Which of the following is true of the respiratory processes?

Possible Answers:

Expiration is usually an active process

Internal intercostals are used for resting expiration

External intercostals are used for forced expiration

The diaphragm contracts during resting inspiration

The diaphragm contracts during forced expiration

Correct answer:

The diaphragm contracts during resting inspiration

Explanation:

The diaphragm contracts during inspiration and relaxes during expiration. External intercostals are used for inspiration, and internal intercostals are used for expiration only if it is forced expiration. Usually expiration is a passive process, unless someone is forcefully exhaling, such as during strenuous exercise.

Contraction of the diaphragm increases the volume of the thoracic cavity, decreasing the pressure. When the pressure in the lungs is less than the atmospheric pressure, air will be drawn into the lungs. When the diaphragm relaxes (passively), the thoracic cavity shrinks and air is expelled.

Example Question #11 : Respiratory Structures And Lung Mechanics

Duchenne Muscular Dystrophy is an X-linked recessive genetic disorder, resulting in the loss of the dystrophin protein. In healthy muscle, dystrophin localizes to the sarcolemma and helps anchor the muscle fiber to the basal lamina. The loss of this protein results in progressive muscle weakness, and eventually death.

In the muscle fibers, the effects of the disease can be exacerbated by auto-immune interference. Weakness of the sarcolemma leads to damage and tears in the membrane. The body’s immune system recognizes the damage and attempts to repair it. However, since the damage exists as a chronic condition, leukocytes begin to present the damaged protein fragments as antigens, stimulating a targeted attack on the damaged parts of the muscle fiber. The attack causes inflammation, fibrosis, and necrosis, further weakening the muscle.

Studies have shown that despite the severe pathology of the muscle fibers, the innervation of the muscle is unaffected.

Duchenne Muscular Dystrophy is usually fatal by age 30. Which of the following is the most likely cause of death for these patients?

Possible Answers:

Traumatic injury

Respiratory failure

Cerebral hemorrhage

Sepsis

Renal failure

Correct answer:

Respiratory failure

Explanation:

Duchenne Muscular Dystrophy is a muscular disorder, so cause of death will be related to muscle weakening. Two main muscles are essential to maintaining the body: the heart and the diaphragm. As the disease progresses to these muscles, causing weakening of the heart and diaphragm, the body begins to deteriorate and cause of death is usually heart failure or respiratory failure when these muscles give out.

Contraction of the diaphragm allows air to enter the lungs. A weaker contraction means less air flow, and eventually leads to respiratory failure.

Example Question #11 : Respiratory Structures And Lung Mechanics

During the process of inspiriation, alveolar pressure initially decreases and then increases. What causes this to occur?

Possible Answers:

An increase in the activity of the internal intercostal muscles results in an initial pressure decrease, while relaxation of the muscles results in an increase in pressure

A decrease in alveolar size results in a decrease in pressure, while continued expiration results in addition of air to the alveoli, causing an increase in pressure

An decrease in the activity of the external intercostal muscles results in an initial pressure decrease, while relaxation of the muscles results in an increase in pressure

An increase in alveolar size results in a decrease in pressure, while continued expiration results in addition of air to the alveoli, causing an increase in pressure

An increase in thoracic volume results in a decrease in pressure, while continued inspiration results in addition of air to the alveoli, causing an increase in pressure

Correct answer:

An increase in thoracic volume results in a decrease in pressure, while continued inspiration results in addition of air to the alveoli, causing an increase in pressure

Explanation:

As inspiration takes place, the diaphragm and the external intercostal muscles contract. This increases the volume of the thorax, which results in a decrease in pressure in the lungs.

As inspiration continues, the addition of air to the alveoli results in an increase in pressure. When alveolar pressure equals atmospheric pressure, inspiration stops.

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